Flexographic plate mounting tape

ABSTRACT

A double sided tape for detachably mounting a flexible printing plate to a flexographic printing press cylinder includes pressure-sensitive adhesive layers on opposed sides of a coextruded multilayer carrier having a core formed of a thermoplastic elastomer containing a uniform distribution of voids and one or more skin layers formed of film-forming thermoplastic polymer. The cellular thermoplastic elastomer is constructed to accommodate compressibility loads imposed during printing and to resist shear and tensile loads imposed during tape removal.

BACKGROUND OF THE INVENTION AND RELATED ART

The present invention relates to adhesive tapes and to methods of makingand using the same. More particularly, the present inventioncontemplates adhesive tapes useful to detachably join elements together,especially at least one flexible element. The tapes are especiallyuseful in printing applications for detachably mounting printing platesin printing presses during printing operations.

In flexographic printing, flexible printing plates carrying the image tobe printed are mounted to printing press cylinders for resilientprinting contact with substrates to be printed. The printing plates areformed of polymeric materials such as rubber or photopolymer, and theyare removably mounted to the steel print cylinders by double sided tapesometimes referred to as "stickyback." In printing applications for bothlabelling and packaging, the printing plates are reused and severaldifferent plates may be used in daily production runs of differentproducts.

The resilient contact between the printing plate and substrate to beprinted is provided at least in part and/or significantly affected bythe compressibility or resiliency of the tape. It is known in the art touse a double sided tape having a carrier containing a flexible foamlayer and opposed layers of pressure sensitive-adhesive. Such doublecoated foam carrier tapes generally provide sufficient compressibilityto assure printing of relatively fine or small indicia. However, thefoam carrier tapes tend to be too compressible for printing ofrelatively large indicia (e.g. indicia having border dimensions ofone-eighth inch or more) and cause excessive ink flow on the substrate.For printing such large indicia, the prior art teaches the use of a lesscompressible tape having a carrier containing a thermoplastic resinlayer such as a polyvinylchloride film layer. Such vinyl carrier tapestend to provide satisfactory printing of large indicia, but they do notprovide the sharpness and fine detail needed in relatively smallindicia. Consequently, printers have tended to inventory and use bothfoam and vinyl carrier tapes in order to attain satisfactory printquality for the full size range of print indicia.

In order to obtain uniform print quality, it is necessary that the tapebe of uniform caliper or thickness. Variations in tape thickness tend tocause print color or intensity differences. Foam carrier tapes havetended to have unacceptable thickness variations, for example, a 20 milthick tape may range from 16 to 24 mils in thickness. Such thicknessvariations yield undesirable print color or intensity differences asnoted above. In extreme cases, thickness variations may prevent properend-to-end abutting alignment of the printing plate around the printcylinder. For example, the two abutting tape edges may be atsufficiently different heights to cause a detectable printing differenceacross the butt joint.

The ease of tape removability from the print cylinder is important toefficient use of the tape and to maintaining good printing productionrates. During installation of the printing plate, it may be necessary toremove and reposition the tape relative to the print cylinder. If thetape tears during its removal for repositioning, the tape is wasted anddiscarded without printing use. Following the printing process, the tapeis removed from the print cylinder in preparation for the mounting of adifferent printing plate. In both cases, the tape should strip cleanlyand separate from the print cylinder by hand manipulation withouttearing or rupturing internal layers so as to leave behind irregularpieces of tape residue such as adhesive and/or tape fragments.Preferably, the tape pieces should strip from the cylinder as anintegral piece without leaving behind tape residue and thereby provide"one piece removability". Prior foam carrier tape constructions tendedto shear or tear in the foam layer during removal from the cylinder andrequired additional time for tape removal and clean-up processing.

G.B. 1,533,431 discloses a laminated double sided tape for mountingflexographic letterpress printing plates including an elastomeric layerhaving rigid-walled frangible bubbles, a dimensionally stablereinforcing layer and opposed outer adhesive layers. The frangiblebubbles are broken with pressure and the wall fragments are retained inthe resulting voids.

A double sided tape for mounting a flexible printing plate to a drum isdisclosed in U.S. Pat. No. 4,574,697. The tape includes a polyurethaneflexible foam applied to a polyethylene terephthalate base film andouter adhesive layers. The tape is indicated to be removable from theprinting plate and drum as an integral sheet, and to be an improvementover vinyl carrier tapes that lack memory and do not retain gauge duringlong printing runs. A similarly constructed tape having a polyethylenefoam layer selected to reduce vibrations is disclosed in European PatentApplication 0 206 760.

U.S. Pat. No. 3,983,287 discloses a printing blanket comprising alaminate including a compressible interior support layer ofincompressible elastomer having dispersed and bonded therein frangiblerigid-walled bubbles that have been broken to impart compressibility.

SUMMARY OF THE INVENTION

As indicated above, the adhesive tape of the present invention may beused to detachably secure elements together. The tape includes a carrierand adhesive layers on opposite faces or sides thereof. The compositionof the carrier layer is selected to achieve desired tape applicationcharacteristics as well as desirable manufacturing or processingbenefits. The tape is especially useful in the mounting of flexographicprinting plates.

The tape carriers are formed of polymeric materials that provide desiredcompressibility and tear resistance characteristics for flexographicprinting applications while at the same time enabling economical tapemanufacturing techniques. More particularly, the carrier comprises amultilayer coextrudate having an internal cellular structure. Thecarrier is formed by the coextrusion of a plurality of resin charges, atleast one of the charges containing a blowing agent to provide thecellular structure.

In preferred constructions, the carrier comprises a closed cell core orcentral layer and at least one skin or outer layer. The core layercomprises a thermoplastic elastomer matrix containing a distribution ofvoids provided by the blowing agent. The skin layer comprises acontinuous non-cellular, film-forming thermoplastic polymer layer thatis preferably substantially free of internal voids as well as voidsalong the interface with the core.

The blowing agent preferably comprises a "so-called" physical blowingagent in that it is thermally blown or expanded upon heating to anelevated temperature, but it does not chemically react to form thecellular structure. (A chemical blowing agent includes a chemicalreaction to generate a gas that forms cells-or voids in the polymermatrix.) The blowing agent expansion temperature may be matched with theextrusion temperature so that the blowing agent does not materiallyaffect the extrusion processing of the core charge. In this manner,coextrusion processing may be used to simultaneously form the core in asingle economical process operation without significant problems due tothe presence of the blowing agent.

In the illustrated embodiment, the tape includes opposed layers ofadhesive and at least one skin layer intermediate the carrier and one ofthe adhesive layers. The skin or skins separate the adhesive from thecarrier layer, and restrict migration of adhesive constituents as wellas the flow of printing ink solvents which may adversely affect theinitial or aged tape properties.

The core provides the majority of the tape strength and compressibilityproperties. The skin smooths the adjacent outer core face which may beroughened excessively by the expansion of the blowing agent. Thesmoothing of the core face is by a leveling of the roughness resultingfrom the blowing process. In this manner, the skin provides the carrierwith a smooth face.

Thermoplastic elastomers having a cellular structure in accordance withthe invention are believed to enable a tape construction of printingcompressibility characteristics suitable for the printing of both smalland large indicia.

The cellular thermoplastic elastomer core layers of the invention alsoprovide tapes of an internal tear strength or cohesive strengthsufficient to enable tape repositionability and removability from aplate holder mounting surface such as a printing cylinder. The tape maybe manually stripped from its installed position around the cylinderwithout layer tearing or tape residue adhering to the cylinder to enablerepositioning of the tape or removal of the plate following completionof the printing process. The tape cleanly separates by manually pullingthe tape from an end or edge thereof to provide "one pieceremovability." The tape also strips cleanly from the printing plate ifit is necessary to reposition the plate during its installation.

The tape carrier outer layer is preferably coextruded in order toenhance the integrity of the multilayer carrier composite as noted aboveand to gain production economies over solvent coating techniques, forexample. The cellular thermoplastic elastomer and the skin layers aretherefore also selected and/or formulated to provide melt andrheological properties enabling coextrusion. The tape including theadhesive layers may also be coextruded with selection and/or formulationof suitable layer properties.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, all of which are highly diagrammatic and do not showthe various elements to scale,

FIG. 1 is a cross-sectional view showing a tape having a multilayercarrier including a core and opposed skin layers which are each joinedto an outer adhesive layer in accordance with the invention;

FIG. 1A is a fragmentary view on an enlarged scale showing a void orcell and a surrounding portion of the polymer matrix;

FIG. 2 illustrates certain steps in the manufacture of the tape as shownin FIG. 1; and

FIG. 3 is a fragmentary elevational view showing a printing cylinderhaving a flexographic printing plate mounted to it using the tape ofFIG. 1, and having the tape pulled away from the right-hand end of theprinting cylinder to show removability of the tape.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to FIG. 1, a tape 10 includes a multilayer carrier 12 having acore or central layer 14 and opposed skin layers 16 and 18. Adhesivelayers 20 and 22 are joined to the skins 16 and 18 at the outer sides ofthe tape 10. Each of the layers of the carrier 12 is integrally bondedto its adjacent neighbor layer by heat bonding or welding during theirmanufacture.

The core 14 has a cellular construction provided by a matrix 14a ofelastomeric material and voids or cells 14b. The voids 14b arepreferably substantially uniformly distributed throughout the matrix 14aand comprise expanded, gas-containing microspheres 14c that form aclosed cell structure. Preferably, the voids 14b do not extend into theskin layers 16 or 18.

In a preferred embodiment, the voids 14b are formed by expandedthermoplastic shell microspheres containing the vapor phase of anorganic liquid. The liquid is evaporated and the volume of themicrosphere is substantially increased during a thermal blowing andexpanding step. Typical expansions may be in the order of 60 times theinitial volume of the liquid containing microsphere. The expansion ofthe microsphere is dependent on the temperature and time of expansionduring the blowing and expanding step.

The shape of the expanded microsphere and void is not critical, butexperience to date indicates a generally spherical shape is attained inthermoplastic block copolymers. Similarly, the size range of theexpanded microspheres is not critical provided the microspheres arecontained within the thickness of the core 14. In a typical core layersample having a thickness of about 20 mils, the expanded microspheresmay range in size from about 11 microns to about 54 microns. The averagemicrosphere size in such sample may be about 27.8 ±5.9 microns at a 95%confidence level.

The void volume of the core 14 may be altered by increasing theconcentration of blowing agent in the core layer charge and/orincreasing the temperature and duration of the blowing and expandingstep. In coextrusion processing, the processing temperature is primarilydetermined by the rheology of the polymers, and the void volume of thecore 14 tends to be varied by changing the concentration of the blowingagent added to the core charge. Typical blowing agent concentrations mayrange from about 1% to 4% by weight based on the weight of the corecharge by using a single screw extruder. It is believed that the blowingagent concentrations may range from 1% to 6% by use of a twin screwextruder.

The blowing agents of interest herein should have relatively highblowing and expanding temperatures to accommodate the extrusiontemperatures of the polymer materials of interest herein. Experience todate indicates that commercially available blowing agents may be usedsatisfactorily with extrusion temperatures in the range of from about150° to about 180° C., and more preferably, from about 155° to about170° C. Higher coextrusion temperatures may cause premature blowing orexpansion of the blowing agent and/or rupture of microspheres and blowholes in the skins so as to result in an inferior product not having therequired compressibility. Lower temperatures may reduce theprocessability of the thermoplastic materials.

The void volume of the carrier 12 is reasonably approximated as theratio of the difference between the measured density of the carrier 12including the skins 16 and 18 and the calculated density of the carrierbased on the densities and thicknesses of the polymer components. Thevoid volume percentage may range from about 10% to about 50%. A typicalvalue is about 35%.

The core 14 is formed of selected thermoplastic elastomers. Thethermoplastic elastomers of interest herein include any block copolymerhaving or containing the triblock structure A-B-A where A represents ablock which is non-rubbery or glassy or crystalline at service or roomtemperature but fluid at higher temperatures, and B represents a blockwhich is rubbery or elastomeric at service or room temperature. Inaddition to the A-B-A triblock structure, other possible structuresinclude radial structures (A-B)_(x) A, where x is greater than 1,diblock structures A-B and combinations of these structures. Theelastomer may comprise from about 60 to 95% rubbery segments by weightand from about 5 to about 40% non-rubbery segments by weight.

The block copolymers of the elastomer form a two-phase system with therubbery and non-rubbery segments being thermodynamically incompatible atservice or room temperature. In accordance with the "domain theory," thenon-rubbery segments tend to agglomerate and form hard domains which actas strong, multifunctional junction points within the elastomer matrixso that the polymer molecules behave as if joined in a cross-linkednetwork. This polymeric arrangement is referred to as "physicalcross-links."

The physical cross-links of the thermoplastic elastomer provide it withproperties similar to those of a vulcanized rubber at room temperature.However, heating causes the domains to soften and the network to loseits strength so that the polymer may flow with application of shearforce. Upon removal of heat, the original elastomer properties areregained.

The thermoplastic elastomer physical cross-links are therefore labileand may be rendered ineffective by processing techniques involving theapplication of heat. In this manner, the multilayer carriers may be hotformed at elevated temperature by conventional hot forming processessuch as coextrusion, sequential extrusion or hot roll coating. The corelayer 14 is integrally joined to the skin layers 16 and 18 during suchprocessing by heat bonding or welding at the adjacent surfaces. However,if necessary, an elastomeric tie coat may be used.

The elastomer is preferably an A-B-A polymer wherein A represents anon-rubbery block which is glassy or crystalline at room or servicetemperature and B represents a rubbery block which is elastomeric atroom or service temperature. Illustrative thermoplastic elastomersinclude elastomers formed of at least one copolymer selected from thegroup consisting of styrene-butadiene-styrene (SBS),styrene-isoprene-styrene (SIS) and styrene-ethylene-butylene-styrene(SEBS).

The thermoplastic elastomer may be blended with a thermoplasticfilm-forming polymer in order to modify its properties. In particular,the resulting blend will tend to be firmer and display a decreasedcompressibility as compared with the thermoplastic elastomer.Polyolefins such as polyethylene may be blended at weight proportionsranging up to 50%, and even 70% with the thermoplastic elastomer toprovide variations in properties such as compressibility.

The skin layers 16 and 18 may be formed of any film-formingthermoplastic polymer which provides smoothing of the core surface andeffectively separates the core layer 14 and adhesive layers 20 and 22.Suitable thermoplastic materials include various hydrocarbon polymerssuch as polyesters, polyurethanes, polyethylenes, polypropylenes,polyamides, and vinyls, and their blends, interpolymers and copolymers.Preferred materials include polyethylene and polypropylene.

The thermoplastic skin layer should preferably be relatively insolublein and unaffected by the printing ink solvents. Preferably, thethermoplastic should have a melt temperature sufficiently close to thatof the thermoplastic elastomer to enable coextrusion of the materialsand formation of a permanent melt bond therebetween.

The thicknesses of the layers of the tape 10 are not critical, and theymay be varied to provide a total tape thickness selected to meetprinting requirements. Thus, the tape 10 may be provided in thicknessesof 10, 15 and 20 mils. In a 20 mil thick tape, the core 14 may have athickness of 16 mils, and the skin layers 16, 18 and the adhesive layers20, 22 each may be 1 mil thick.

Although the total thickness of the tape is not critical, uniformcaliper should be maintained for a given tape thickness. Experience todate indicates that tape constructions in accordance with the inventionmay be provided with less than a 10% variation in thickness. Thus, a 20mil thick tape may vary less than ±1 mil. Such thickness variation hasbeen found acceptable for flexographic printing tape applications and tonot yield a butt joint which is detectable in the printed product.

The core 14 comprises a thermoplastic elastomer as noted above.Presently, the preferred elastomers are SBS, SIS, SI, S(IS)_(x) and SEBSblock copolymers and mixtures thereof sold by the Shell Chemical Companyunder the designations Kraton D1102, D1107, D1111, D1112, D1117, D1125and D4141 and Kraton G1650. These elastomers have hardness, Shore Avalues ranging from 32 to 75 and styrene/rubber ratios ranging from14/86 to 29/71.

As indicated above, it is not necessary that one or both of the skinlayers 16 and 18 be used in the absence of a need to prevent migrationof the constituents of the adjacent layers. Further, it is not necessarythat the skin layers 16 and 18 be of the same composition. In additionto providing a barrier and separation function, the skin layers shouldalso be resistant to printing ink solvents. Examples of suitable skinlayer materials include a low density polyethylene sold under thedesignation Dow LDPE 993 by Dow Chemical.

The adhesive layers 20 and 22 need not be of the same composition. Theuse of different adhesive compositions may be advantageous since thelayers 20 and 22 generally will not be adhered to like materials inprinting applications. However, it is often convenient to use the sameadhesive for production purposes. The specific adhesive composition isnot critical, and many acrylic and rubber based pressure-sensitiveadhesives may be adapted for use herein. Suitable acrylic basedadhesives are available from Avery Chemical and Ashland Chemical. Rubberbased adhesives may be based on natural or synthetic rubbers. Syntheticrubber pressure-sensitive adhesives may be formulated using Kraton D andG series thermoplastic elastomers by Shell Chemical Company with theaddition of tackifiers and other additives as is well known in the art.

Referring to FIG. 2, the multilayer carrier 12 may be prepared bycoextrusion processing using a multifeed coextrusion ABA type die 24having a die opening 24a corresponding with the dimensions of thecoextruded core and skin layers. The die is supplied with skin and corecharges by associated extruders (not shown). The shaped skin and corecharges are joined just prior to exiting through die opening 24a in aknown manner.

The core charge comprises a thermoplastic elastomer mixed with a blowingagent. For example, the elastomer may be Kraton D1112 which is a SISblock copolymer containing 60% SIS and 40% SI. A suitable physicalblowing agent is sold under the designation Expancel® DU-091 by CascoNobel AB of Sweden. The blowing agent is described as comprising a dropof liquid isobutane encapsulated by a gas-proof thermoplastic polymericshell or microsphere. Upon heating, the shell is indicated to expand tomore than 60 times its initial volume. The blowing agent is mixed withthe elastomer at a concentration of 3% based on the weight of theelastomer. In addition, a plasticizer such as Shellflex® 371 oil may beadded at a concentration of 3% based on the weight of the elastomer.

The skin charge may comprise a low density polyethylene such as theabove mentioned Dow LDPE 993 by Dow Chemical.

The core and skin charges are heated and fed to the die 24 with littleor no blowing and expanding of the blowing agent. The heating of thecharges continues in the die 24 to a maximum or coextrusion temperatureof about 160° C. for the above described materials. Substantially all ofthe expansion of the blowing agent is believed to occur just as thepolymer begins to flow out of the opening of the die 24 and the pressureapplied to the polymer is reduced. In this manner, the blowing agenteffects on the core charge are minimal during the coextrusion processingthereof, since it is only at the exit from the extrusion die, e.g.within several inches of the die opening, that substantial expansion ofthe blowing agent occurs. If expansion of the blowing agent occursearlier in the extrusion process, there is a tendency for themicrospheres to blow out through the skin layers sometimes resulting ina very rough outer layer surface. Excessive pre-blowing can result in aneffective loss of the cellular structure.

Prior to thermal blowing or expansion, the blowing agent effects on thecharge and process may be likened to those resulting from the presenceof an inert filler. Upon expansion or blowing of the agent as theextrusion exits from the die, the benefits of relatively larger sizecells are obtained since the blowing agent may increase in size by anorder of magnitude, i.e. 10 to 50 times larger or more.

The coextrudate emerging from the die 24 is contacted with a chill roll(not shown) in a conventional manner. This is believed to endsubstantially all blowing and expansion, and to fix the matrix and voidstructure of the core 14. The film may be contacted with furtherprocessing rolls in a conventional casting type coextrusion process.

The carrier 12 may be wound without a liner and stored for laterprocessing following coextrusion and suitable cooling of the coextrudatesince the skins 16,18 are typically non-blocking. The carrier 12 maythereafter be subsequently combined with suitable adhesive layers andrelease liners.

For convenience, a continuous production line is shown in FIG. 2.Following coextrusion, the carrier 12 is conveyed to a first adhesivecoating station "A" for applying the adhesive layer 20 to the outsidesurface of the skin layer 16. A hot roll coater 26 is used to combineadhesive layer 20 with the carrier 12. The roll coater 26 includes asits major elements a casting roll 28 for delivering a coating ofadhesive melt to a transfer roll 30 which cooperates with a back-up roll32 to apply the adhesive to the surface of the skin layer 16. Theadhesive melt is picked-up from a pan supply 34.

The adhesive layer 20 is cooled with suitable chill rolls (not shown) orother means, and the combined carrier 12 and layer 20 are inverted asthey pass through a web turnover 36. The combined carrier 12 and layer20 are conveyed to a second adhesive coating station "B" where adhesivelayer 22 is applied to the outside surface of the skin layer 18 by asecond roll coater 38 to complete the tape 10. As shown, the roll coater38 includes a casting roll 40, transfer roll 42 and back-up roll 44which cooperate to apply adhesive melt from adhesive pan supply 46 tothe skin layer 18 to form the adhesive layer 22. The adhesive layer 22is cooled with chill rolls (not shown) or other means, and the completedtape 10 is taken-up on roll 48 with the insertion of a suitable releaseliner to separate the wound adhesive layers. Typical release liners maycomprise double coated, differential release liners or the use of twosingle coated release liners. The release liner may include a releasecoating or low adhesion coating, e.g. silicone, as are known in the art.Examples of such release coatings are set forth in U.S. Pat. No.2,985,554.

The tape 10 may be manufactured in 54 inch wide rolls and subsequentlyslit to a more narrow width suitable for mounting flexographic printingplates. Such tape is typically slit to a 20 inch width or less. The 20inch width corresponds with the width of many print cylinders. Thus, theprinter or user of the tape may use a "right angle wrap" by winding thetape in its lengthwise or machine direction about the print cylindercircumference at a right angle to the cylinder axis or a "lengthwisewrap" by extending the tape in its lengthwise direction along the lengthof the print cylinder and wrapping the tape in its widthwise or crossmachine direction about the print cylinder circumference. In the case oflarger print cylinders, for example, 60 inch wide cylinders used in somepackaging applications, a lengthwise wrap is customarily used. In allcases, a butt joint is formed at adjacent edges of the wrapped tape byinitially overlapping the adjacent portions of the tape, cutting bothplies of the overlapped tape with a single cut and stripping away theoverlying tape portions to form a butt joint.

Referring to FIG. 3, the use of the tape 10 to mount a flexographicprinting plate 50 to a print cylinder 52 is illustrated. The printcylinder 52 is of a size frequently used in label printing applications.It has a 20 inch width and a diameter of slightly less than about 3.5inches. The tape adhesive must be sufficiently strong to hold theflexographic plate 50 tightly to the relatively small circumference ofthe print cylinder 52.

The print cylinder 52 is initially cleaned so that its steel cylindersurface 52a is free of printing ink, solvent and debris. The tape 10together with its protective liner (not shown) should be sufficientlysupple to allow it to be hand wrapped smoothly against the surface ofthe print cylinder without folds, bubbles or other non-conformingirregularities.

In this instance, a right angle wrap is used and the tape together withits associated release liner are cut to a length slightly greater thanthe circumference of the print cylinder 52. The exposed adhesive layer20 of the tape 10 is pressed against the cylinder surface 52a andaligned with one of the cylinder edges to provide a right angleorientation with respect to the cylinder axis "L". If the tape installeris not satisfied with the alignment, the tape may be pulled from thecylinder surface 52a cleanly and without separation as shown at theright-hand edge of the cylinder, and repositioned. During suchinstallation, the adhesive layer 22 is protected by the liner (notshown).

The lengthwise end portions of the tape overlap, and a single cut ismade through the overlying portions to form a butt joint 54 whichextends along the length of the print cylinder 52. As compared withprior art foam carrier tapes, the thermoplastic core 14 of the tape 10is more easily cut to form a clean edge needed in a uniform butt joint.Often, prior art foam carrier tapes tear during cutting and leave raggededges which do not form uniform joints.

The liner is then removed from the installed tape to expose the adhesivelayer 22. The flexographic printing plate 50 is wrapped about thecircumference of the cylinder and pressed against the adhesive layer 22to tightly secure it to the cylinder surface 52a. The plate 50 is sizedfor use on a particular print cylinder and the ends of the plate areplaced together to form a butt joint 56. The mounted printing plate 50is now ready for printing operation.

During the printing operation, ink is applied by an inking roll (notshown) to the raised surfaces of the printing plate 50 correspondingwith the indicia to be printed. The substrate to be printed is thenresiliently contacted by the printing plate 50. The plate 50 includesboth large indicia such as rectangle 58 having edge dimensions in theorder of several inches and small indicia such as bar code 60 having anedge dimension of less than about several hundredths of an inch. Theresilient characteristics of the contact with the substrate is providedin part or affected by the resiliency or compressibility of the tape 10.The tape 10 has a printing resiliency or compressibility intermediatethat of prior art foam and resin film carrier tapes, and it thereforeenables acceptable quality printing of both large and small indicia.

After the completion of the printing process, the printing plate 50 isseparated from the adhesive layer 22 without tearing or internallayer-shearing of the latter. The tape 10 is then manually stripped fromthe print cylinder 52 without tearing or internal layer-shearing of thetape so as to leave the cylinder surface 52a free of tape residue.

In accordance with the invention, a tape was constructed having amultilayer coextruded carrier comprising a core layer charge of KratonD1112, 3% Expancel® DU-091 and 3% Shelflex 371 oil. A polyethylene skinof Dow LDPE 993 was provided on each side of the core. A rubber basedadhesive charge of the following formulation was prepared.

    ______________________________________                                        Kraton D1102 (SBS)      17%                                                   Kraton D1117 (SIS)      17%                                                   Rosin ester tackifier   45%                                                   Mineral oil plasticizer 10%                                                   CaCO.sub.3              10%                                                   Antioxidant              1%                                                   ______________________________________                                    

A layer of adhesive was applied to each skin layer of the carrier by hotroll coating as described above. The core layer was 16 mils thick andeach of the other layers was 1 mil thick to provide a 20 mil thick tape.The thickness variation of the tape was less than ±1 mil. This tape isidentified as Example 1 below and compared with an experimental tape andcommercially available tapes having the constructions identified asComparative Examples 1C through 5C hereinafter.

Comparative Example 1C comprises an experimental tape having amultilayer coextruded carrier including a core layer charge of KratonD1125 which is SIS thermoplastic elastomer and a skin layer charge ofthermoplastic polyurethane sold under the designation Pelethane by DowChemical. A skin was provided on each side of the core layer. As inExample 1, the core layer was 16 mils thick and each of the other layerswas 1 mil thick to provide a 20 mil thick tape. This comparative exampledoes not include a blowing agent, and the core and skin layers aresubstantially void free and continuous. Adhesive layers were applied toeach face of the carrier using the same adhesive and processing asdescribed above in Example 1.

Comparative Example 2C includes a 2.5 mil spun non-woven central layerand opposed 7.5 mil foamed adhesive layers, one layer being an acrylicadhesive and the other being a rubber adhesive.

Comparative Example 3C is a four layer construction including a 3 milpolyvinylchloride central film layer secured by an acrylic adhesive to a13 mil ethylvinyl acetate foam layer. Opposed outer adhesive layers areformed of acrylic and rubber adhesives respectively.

Comparative Example 4C includes an ethylvinyl acetate foam layer havingpolyethylene skins and opposed adhesive layers of acrylic and naturalrubber.

Comparative Example 5C includes a 6.2 mil thick polyvinylchloridecarrier film and opposed natural rubber based adhesive layers.

In accordance with the invention, the core layer 14 of the tape 10provides both the desired tear resistance and compressibility. Theseproperties are compared with the above described tapes.

The tear resistance of the central or core layer was measured for eachof the above tapes using a modification of the test procedure of ASTM D3574, Test F. In the case of relatively compressible tapes (e.g. tape3C), such central or core layer is often formed of a cellular orotherwise compactable or collapsible construction, e.g. layer 14 of thetape 10. However, the central layer may be formed of a relativelycontinuous film material such as the polyvinylchloride film of tape 5C.In accordance with the test, the central layer is separated from theadjacent layers along a small portion of the tape length at one endthereof. The outer adhesive tape layers are adhered to masking tapestrips having terminal ends extending beyond the ends of the separatedtape layers and folded over to form gripper tabs. The tear strength ofthe central layer is measured by holding one of the masking tape grippertabs stationary and pulling the other tab at an angle of about 180° anda speed of 12 inches per minute to cause or attempt to cause an internaltear failure in the central or core layer. The tear strength is reportedfor tapes having a tear-type failure in lbs./in. If the tape does nothave a tear failure (e.g. masking tape or adhesive tape failed insteadof the interior layer), then the test is reported as "n/a" since norelevant tear failure occurred.

The compressibility of a tape is indicated by the coefficient ofcompression ("COC") as measured using a modification of the testprocedure of ASTM D3574, Test C. In this test, the tape is engaged by aweighted compression foot of a thickness gauge. Herein, a generallyU-shape wire having a length of about one inch and a diameter of about0.038 inches is placed between the compression foot and the core layerto better simulate the loads applied by the relatively small dimensionsof the printing plate indicia of the flexographic printing plate. Thetape thickness is measured with compression foot loads of 250, 500,1000, 2500 and 4540 grams, and the percent thickness based on the 250gram load is calculated as follows. ##EQU1## The thickness percent isplotted against the log of the applied weight, and the COC is equal tothe absolute value of the slope of the curve. Thus, a relatively highCOC value indicates a more compressible material.

                  TABLE I                                                         ______________________________________                                                 NOMINAL                COEFFICIENT                                   EXAMPLE  THICK-     TEAR        OF                                            NUMBER   NESS.sup.1 STRENGTH.sup.2                                                                            COMPRESSION                                   ______________________________________                                        1        20         n/a         61                                            1C       20         n/a         28                                            2C       20         1.9         37                                            3C       20         4.5         62                                            4C       20         9.8         46                                            5C       10         n/a         34                                            ______________________________________                                         .sup.1 mils                                                                   .sup.2 lbs./inch                                                         

The tape in accordance with Example 1 has satisfactorily enabledprinting of large and small indicia so as to indicate preferred printingcompressibility, The tape of the present invention also has a relativelyhigh cohesive or internal strength indicated by its high tear resistanceso as to enhance its repositionability and one piece removability.

It should be evident that this disclosure is by way of example and thatvarious changes may be made by adding, modifying or eliminating detailswithout departing from the fair scope of the teaching contained in thisdisclosure. The invention is therefore not limited to particular detailsof this disclosure except to the extent that the following claims arenecessarily so limited.

What is claimed:
 1. A double sided tape for detachably mounting aflexible printing plate to a printing press plate holder, said mountedprinting plate being adapted to apply printing ink to a plurality ofworkpieces to be printed during printing operation, and to then beunmounted by detaching the printing plate from the holder, said tapecomprising a hot formed multilayer carrier including a core layer formedof a thermoplastic elastomer matrix containing a substantially uniformdistribution of voids generated by a blowing agent comprising thermallyexpandable microspheres and at least one skin layer formed of afilm-forming thermoplastic polymer, said core layer having a void volumedetermined by the proportion of blowing agent in the core layer and bythe degree of expansion thereof, said voids having a size substantiallycorresponding with the size of the expanded microspheres and being atleast an order of magnitude larger than the microspheres of said blowingagent prior to expansion, said core layer having a compressibility and atear strength which respectively accommodate compressive loads imposedduring printing and tear or shear loads imposed during tape removal,said tape also including first and second pressure-sensitive adhesivelayers respectively on opposed faces of said carrier, said tape beingadapted to cause said printing plate during press operation to applyprinting ink to a succession of workpieces to be printed at a degree ofcompression of said carrier which is substantially predetermined by thevoid volume of said core layer, and said core layer during tape removalproviding sufficient tear strength to cleanly strip and separate saidtape from said holder.
 2. A method as in claim 1, wherein saidthermoplastic elastomer matrix comprises an A-B-A, (A-B)_(x) A, or A-Bblock copolymer, or mixtures thereof, wherein A represents a non-rubberyblock which is glassy or crystalline at room or service temperature andB represents a rubbery block which is elastomeric at room or servicetemperature.
 3. A tape as in claim 2, wherein said thermoplasticelastomer is formed of at least one block copolymer selected from thegroup consisting of styrene-isoprene-styrene,styrene-ethylene-butylene-styrene and styrene-ethylene-butylene-styrene.4. A tape as in claim 3, wherein said film-forming thermoplastic polymeris a member selected from the group consisting of polyethylene andpolypropylene.
 5. A tape as in claim 3, wherein said carrier has acoefficient of compressibility of about 61 and a Shore A hardness in therange of from about 25 to about
 90. 6. A tape as in claim 3, whereinsaid carrier has a tear resistance in excess of the adhesion of theflexible printing plate to the printing plate holder.
 7. A tape as inclaim 3, wherein at least one of said pressure-sensitive adhesive layersis a repositionable adhesive.
 8. A tape as in claim 3, wherein saidpressure-sensitive adhesive layers are formed of an adhesive selectedform the group consisting of acrylic based adhesive and rubber basedadhesive.
 9. A tape as in claim 8, wherein said first and secondpressure-sensitive adhesive layers are formed of the samepressure-sensitive adhesive.
 10. A method as in claim 3, wherein atleast one of said pressure-sensitive adhesive layers is formed of athermoplastic elastomer and a tackifier, and said skin layer preventsmigration of said tackifier from said at least one adhesive layer tosaid core layer.
 11. A tape as in claim 3, wherein said core layer isformed of a physical blend of said thermoplastic elastomer and a secondfilm-forming thermoplastic polymer.
 12. A tape as in claim 11, whereinsaid second thermoplastic polymer is a polyolefin which reduces thecompressibility of the core.
 13. A tape as in claim 3, wherein saidthermoplastic polymer forming said skin layer is substantially free ofvoids and is resistant to printing ink solvent and said skin layerprotects said core.
 14. A tape as in claim 1, wherein said blowing agentmicrospheres include an expandable thermoplastic shell containing aliquid that is vaporized during expansion and substantially containedwithin the expanded thermoplastic shell.
 15. A tape as in claim 1,wherein said carrier is hot formed at a forming temperature and saidblowing agent has an expansion temperature selected to enable hotforming of said carrier with expansion of said blowing agent occurringat substantially the end of the carrier forming process withoutmaterially affecting the manipulative steps thereof.
 16. A tape as inclaim 15, wherein said voids have an average size of 27.8±5.9 microns.17. A tape as in claim 15, wherein said voids range from about 11microns to about 54 microns in size.
 18. A tape as in claim 15, whereinsaid core has a void volume in the range of from about 10% to about 50%.19. A tape as in claim 2, wherein said blowing agent microspheresinclude an expandable thermoplastic shell containing a liquid that isvaporized during expansion and substantially contained within theexpanded thermoplastic shell, said voids range from about 11 to about 54microns in size and said core has a void volume in the range of fromabout 10% to about 50%.
 20. A double sided tape for detachably mountinga flexible printing plate to a printing press plate holder, said mountedprinting plate being adapted to apply printing ink to a plurality ofworkpieces to be printed during printing operation and to then beunmounted by detaching the printing plate from the holder, said tapeincluding a multilayer carrier comprising a coextrusion of a firstcharge of an elastomer containing a blowing agent comprising thermallyexpandable microspheres for forming a core layer and a second charge ofa film-forming thermoplastic polymer for forming at least one skinlayer, said carrier being coextruded at an extrusion temperature andsaid blowing agent having an expansion temperature selected to enablecoextrusion of said carrier with expansion of said blowing agentoccurring at substantially the end of the extruding process, withoutmaterially affecting the manipulative steps thereof, to provide saidcore as a matrix of said elastomer containing a substantially uniformdistribution of voids formed by the expanded microspheres of saidblowing agent, said tape also including first and secondpressure-sensitive adhesive layers respectively on opposed faces of saidcarrier.
 21. A tape according to claim 20, wherein said elastomer ofsaid first charge is a thermoplastic elastomer, said core layer has avoid volume determined by the proportion of blowing agent in the firstcharge and by the degree of expansion of the blowing agent, and saidvoids have a size substantially corresponding with the size of theexpanded microspheres and being at least an order of magnitude largerthan the microspheres of said blowing agent prior to expansion.
 22. Atape according to claim 21, wherein said carrier is coextruded using acoextrusion die and said expansion temperature is selected to causeexpansion of said blowing agent substantially as said carrier exits saiddie.
 23. A tape according to claim 22, wherein said tape is adapted tocause said printing plate during press operation to apply printing inkto a succession of workpieces to be printed at a degree of compressionof said carrier which is substantially predetermined by the void volumeof said core layer, and said core layer during tape removal providingsufficient tear strength to cleanly strip and separate said tape fromsaid holder.
 24. A tape as in claim 22, wherein said voids range fromabout 11 microns to about 54 microns in size.
 25. A tape as in claim 24,wherein said core has a void volume in the range of from about 10% toabout 50%.
 26. A tape as in claim 20, wherein said blowing agentmicrospheres include an expandable thermoplastic shell containing aliquid that is vaporized during expansion and substantially containedwithin the expanded thermoplastic shell, said voids range from about 11to about 54 microns in size and said core has a void volume in the rangeof from about 10% to about 50%.
 27. A tape according to claim 23,wherein said thermoplastic elastomer comprises an A-B-A, (A-B)_(x) A, orA-B block copolymer, or mixtures thereof, wherein A represents anon-rubbery block which is glassy or crystalline at room or servicetemperature and B represents a rubbery block which is elastomeric atroom or service temperature.
 28. A tape as in claim 27, wherein saidthermoplastic elastomer is formed of at least one block copolymerselected from the group consisting of styrene-butadiene-styrene,styrene-isoprene-styrene, and styrene-ethylene-butylene-styrene.